1. Field of the Invention
This invention relates generally to glass compositions having improved melting and refining characteristics and, more particularly, to methods of adjusting a glass composition to lower the temperature(s) of the melting and/or forming viscosities without substantially changing the temperature(s) of the bending and/or annealing viscosities of the glass. The invention also relates to glass articles made from the glass compositions.
2. Technical Considerations
Glass manufacturers melt glass batch materials and refine the molten glass to form glass articles. For example, in a conventional float glass process, glass batch materials are heated in a furnace or melter to form a glass melt. The glass melt is poured onto a bath of molten tin, where the glass melt is formed and continuously cooled to form a float glass ribbon. The float glass ribbon is cooled and cut to form solid glass articles, such as flat glass sheets. The particular batch materials used and their relative amounts are selected based on the desired properties of the glass articles. Exemplary glass batch compositions are disclosed in U.S. Pat. Nos. 5,071,796; 5,837,629; 5,688,727; 5,545,596; 5,780,372; 5,352,640; and 5,807,417, just to name a few.
As will be appreciated by one of ordinary skill in the glass manufacturing art, glass composition properties can be defined based on their temperature and viscosity characteristics. For example, the “melting temperature” of a glass is conventionally defined as the temperature at which the glass has a viscosity of 100 poises, which is conventionally referred to as the temperature of the “log 2”viscosity (i.e., the logarithm of the viscosity of the glass in poises is 2). Similarly, the “forming temperature” (log 4 viscosity), “bending temperature” (log 7.6 viscosity), “annealing temperature (log 13 viscosity), and “strain point” (log 14.5 viscosity), are conventionally defined as the temperatures at which the logarithms of the glass viscosity in poises are 4, 7.6, 13, and 14.5, respectively. The “liquidus temperature” is that temperature at which the glass begins to devitrify, which can cause undesirable haziness in the glass product. The difference between the forming temperature and the liquidus temperature is known as the “working range”. It is generally desirable to have a working range spanning more than 40° F. (22° C.).
Glass fabricators purchase flat glass sheets from glass manufacturers and process these glass sheets into various commercial products, such as architectural windows, mirrors, shower doors, automotive windows, insulating glass units, etc. Typically, this processing includes heating the flat glass sheets to bend the sheets and then controllably cool the sheets to anneal, temper, or heat strengthen the sheets. The bending, tempering and/or annealing temperatures for a particular type of glass are important economic factors in the fabrication process and cannot be easily changed without substantially altering the existing fabrication process, which would be expensive and time consuming.
Due to increased tonnage and quality demand for flat glass products, flat glass manufacturers are under pressure to increase their glass production while reducing the cost of manufacturing the glass. Many glass manufacturers are operating their glass furnaces at higher and higher throughput and temperatures to meet the increased demand for glass. However, this need to increase glass production has resulted in several problem areas. For example, the operating temperature of a conventional flat glass furnace is typically on the order of 2850° F. (1564° C.). As more glass batch material is processed through the furnace, more fuel is required to melt the increased amounts of glass batch materials in a shorter time period. This increased fuel usage adds significantly to the production cost of the glass sheets or articles and results in a decreased thermal efficiency for the melting operation. Further, running the melter at increased throughput and at elevated temperatures can also damage the melter refractories, such as by causing thermal and/or chemical damage to the silica crowns and breast walls, which can lead to premature failure or collapse of the melter superstructure and solid defects in the glass.
Therefore, it would be advantageous to provide glass manufacturers with a method of adjusting a glass composition (and thus the batch materials from which it is made) to provide a lower melting point to decrease fuel usage and potential damage to the melter while maintaining substantially the same bending and annealing temperatures as the starting glass composition.